This seems to be an ongoing issue that is still reproducable in the latest Ubuntu Vbox 6.x.
The default NAT Network range is usually 10.0.2.0/24. If you change this range it does not seem to work properly.
Say we change the range to 10.50.1.0/24
If you get a new lease you will find that you get an IP from the old range but the default gateway is from the new range.
........